Another vote for the Tokina 12-24/4. You might be able to find a used copy of the original version for perhaps $300-350, but they hold their value pretty well since most people like them a lot. Tokina introduced a new version just this year... seems mostly just some new coatings (which probably just means they now market the identical lens to the one they make for Pentax and Nikon). So the earlier one that originally sold for $500 is now discounted a bit, whether new or used. Very well made, too... More solid feeling than the Canon 10-22 that sells for a couple hundred dollars more (and doesn't even include the lens hood).
Sometime do yourself a favor and get a set of macro extension tubes (Kenko, $160... Adorama, $100). Use those mostly with your 28-135 for macro work... Even though it focuses pretty darned close already.
The 12-24 and 28-135 make a really nice pair of lenses, compliment each other very well, and actually cover a wider range than most film shooters ever owned in their entire lifetimes, back in the good/bad old days.
On your camera... a 10mm lens is an "ultra wide". A 12mm is "very wide". 15mm to 20 or 21mm would be "wide angle". 24mm to 35mm would be a "normal" or "standard". 50mm and 85mm are "short telephotos". 135mm to 150mm are "moderate telephotos". 200mm is a "long telephoto" And anything greater than 200mm is getting into "super telephotos".
Most macro lenses happen to be short to moderate telephotos. It's just what works best, giving you enough room to not cast shadows on your subject, not scare away living thngs, etc. Most people starting out (with a crop sensor camera like your 40D) I'd recommend a 60mm to 105mm as a starting point that's fairly easy to use. Notice that your current zoom lens covers that range nicely. Now, for maximum detail and sharpness, most true macro lenses are not zooms, but primes - which is another way of saying a "fixed focal length". But, your zoom is a great place to start and learn what you like.
